1. Field of the Invention
The present invention relates to a hard material bit for a rotary-percussion tool, in particular for a rotary-percussion drill having a diameter from 4 mm to 45 mm and used in a rotary percussion power tool. The present invention also relates to a rotary percussion power tool provided with a hard material bit.
2. Description of the Prior Art
Usually, the rotary-percussion drills of the type described above and which are used in rotary-percussion power tools, have a chisel-shaped hard metal plate that is secured in the shaft end of a drill. The hard metal plate is brazed in a receiving slot extending diametrically through the shaft end along its bottom surface and two, diametrically opposite attachment surfaces. Large variations of temperatures, which are observed during assembly works and percussion drilling, result, because of different heat expansion coefficients of the hard material bit and the shaft end, in high internal stresses in a hard solder that connects the hard material bit with the shaft end, and this can lead to failure of the tool.
German Publication DE102089631 discloses a plate-shaped hard material bit having a bottom surface, which is located opposite the cutting edges and two attachment surfaces extending transverse to the bottom surface. Between the bottom surface and the two attachment surfaces, there are provided, respectively, two chamfers associated with inner rims of the receiving slot formed in the shaft end. The free surfaces, which extend perpendicular to both the bottom surface and to the attachment surfaces, pass, without chamfers, into the bottom surface.
German publication DE4419641 discloses a compact hard material bit with cutting edges which is butt-brazed to the base surface of the drill shaft with its bottom surface without chamfer. German publication DE10006936 discloses a rotary-percussion drill specifically designed for drilling holes in a reinforced concrete and provided with an X-shaped hard material bit formed of a sintered material and brazed in a matching X-shaped mortise formed in the shaft end over two, diametrically opposite attachment surfaces and a bottom surface and which is further butt-brazed to the base surface of the shaft end along sectors of two diametrically opposite free surfaces along the bottom surface. The matching X-shaped hard material bit according to DE10006932 has, in the bottom surface opposite the cutting edges, a chamfer only on the attachment surfaces.
German publication DE4442266 discloses a cross-shaped hard material bit with a uniformly extending chamfer of the same width that is provided in the bottom surface.
An object of the invention is a hard material bit for a rotary-percussion tool for drilling holes in reinforced concrete and having an increased service life.